This invention relates to a centrifugal fluid pump assembly for pumping a medical fluid such as blood.
A centrifugal blood pump is one type of pump used in artificial heart units. A centrifugal blood pump includes a housing having a blood inlet and a blood outlet, and an impeller which rotates within the housing for feeding blood by centrifugal force upon rotation thereof. One of the significant parameters for grasping that the blood pump functions correctly on a living organism is the flow rate. The flow rate may be measured directly using a sensor for exclusive use such as an ultrasonic flow meter. However, the direct measurement of the flow rate is inferior in terms of stable continuous monitoring for a long period of time. Further, an increase of the number of parts leads to increase the size of the apparatus. Therefore, a system is expected wherein the flow rate is measured indirectly from the motor speed (rotation speed) of the pump, electric current flowing through the motor (motor current), the viscosity and the specific gravity of blood and/or like parameters without the provision of a flow meter.
In an indirect measurement system, the flow rate can be calculated by correcting a value calculated regarding the flow rate as a function of the motor speed and the motor current with a viscosity factor. In particular, the flow rate is calculated from the speed of rotation and the motor current of the motor which can be measured directly. In this instance, it is possible to determine a correlation between the motor speed, the motor current and the flow rate in advance and use an approximate expression calculated from the correlation. The approximate expression can be represented, for example, as the following expression (1):                     i        =                              ∑                          k              =              0                        2                    ⁢                                           ⁢                                    ∑                              l                =                0                            2                        ⁢                                                   ⁢                                          ∑                                  m                  =                  0                                2                            ⁢                                                           ⁢                                                C                                      k                    /                    m                                                  ⁢                                  V                  k                                ⁢                                  n                  l                                ⁢                                  q                  m                                                                                        (        1        )            where cklm is a coefficient calculated by the method of least squires, i [A] is the motor current, v [mPa·s(cP)] is the viscosity, n [rpm] is the speed of rotation, and q [L/min] is the flow rate.
Where such a polynomial as given above is used, however, if the degree of approximation is lowered, then the accuracy basically in the overall region drops including data points (sample points used in the terminology of interpolation). On the other hand, if the degree of approximation is raised, then unnecessary oscillation which is a drawback in polynomial interpolation takes place, and also this gives rise to a drop the accuracy except data points.
In order to prevent such a drop of the accuracy as described above, it is possible to prepare and refer to a large amount of data in a table to use a flow rate in the conditions nearest to a given motor speed and a given motor current as an estimated value. However, according to this system, an increased area for storing data is required, and an increase of processing of searching the storage area increases a load upon a controller for the pump. Thus, if the controller is a portable type, it is difficult to display the flow rate on the real time basis on the controller.
Also where spline interpolation is used as the interpolation, there is a problem that a great amount of calculation is required although the problem of the accuracy is solved.
It is an object of the present invention to provide a centrifugal fluid pump assembly which has a flow rate calculation function by which the fluid flow rate (discharge) can be calculated readily and with certainty without using a flow meter.
It is another object of the present invention to provide a magnetically levitated centrifugal pump wherein minimum discrete data with which a necessary accuracy can be satisfied are stored without having a relational expression between the motor speed, motor current, viscosity and flow rate, and their stored data which are proximate to an motor current value and so forth of an object of calculation are used to calculate the flow rate.